United States believes Iran is developing a nuclear bomb

Iran has never been shy about it’s desire to destroy the Western powers it sees as promoting global hegemony and threatening their way of life. That’s why the United States feels such concern over the notion that Iran is developing a nuclear bomb. They’ve never come out and said as much but there are strong signs largely related to the acquisition of materials needed to create such a powerful device.
The thinking generally goes that if Iran develops a nuclear bomb it will almost immediately use it on Israel, a country it seems to harbor more anger for than it does for the United States. Israel is a close ally of the US and so a nuclear bomb dropped there would be an enormous disaster for all concerned. Thus there is an enormous strain between US and Iran relations.